Shin Kidou Senki Gundam W: Battlefield of Pacifists

After a generation of warfare, the Gundam boys finally brought peace between Earth and the colonies when they helped defeat the Oz and White Fang armies. Now, out of the rubble, the surviving soldiers are re-emerging and clustering together into new factions. Heero Yuy has become a vigilante pacifist, tracking down weapons stashes and destroying them, while Relena, former Queen of the Earth Sphere, is now trying to maintain the peace politically. But an unmanned weapons factory has been discovered somewhere near Mars, and now the Gundam boys are going to have to dust off their mechs for one more battle if peace is to stand a chance of lasting.

Battlefield of Pacifists is one of the possible events that bridge the gap between the anime series and the OVA, Endless Waltz. Other manga that also present such possibilities are Blind Target and Ground Zero.

Here are other titles that are related to this one:

Shin Kidou Senki Gundam W (is the Main Story)
Shin Kidou Senki Gundam W: Endless Waltz (is also the Main Story)
Shin Kidou Senki Gundam W: Blind Target (is the Alternate Story) 
Shin Kidou Senki Gundam W: Ground Zero (is also the Alternate Story)